The invention relates to drag reduction of waterborne vehicles and more particularly to a water ablative paint coating having drag reducing properties.
Generally, the relative movement of a vehicle through a fluid, be it air or water, a surface vehicle or submerged, creates various drag forces on the vehicle's surface which impedes its forward movement. One form of drag is created by shear forces of the viscous fluid in the boundary layer immediately adjacent to the skin surface of a waterborne vehicle.
Another form of drag results from turbulent flow of the fluid as the vehicle passes through the fluid medium. At the bow of the vehicle, the fluid flow around the frontal contour is substantially laminar with no apparent evidence of roiliness. As the flow continues aft, the laminar flow becomes turbulent, contributing to drag or skin friction. The boundary layer separates from the vehicle skin thus creating a void or cavity in the fluid with a suction like effect. The result is that the vehicle cannot benefit or recover a thrust from a pressure rise at the tail or stern. In summary, all these drag forces together contribute to impeding forward motion of the vehicle through the fluid medium, thus causing a decrease in speed and/or an increase in power requirements.
Various studies, made in the past, have shown that polyethylene oxide and certain polyacrylamide resins possess properties that minimize drag or skin friction. With this knowledge, various prior art proposals have been made in order to provide a means whereby these drag reducing polymers may be introduced into the boundary layer. These proposals have included the injection of concentrated drag reducing polymer solutions into the boundary layer near the vehicle skin, and the application of drag reducing polymer coatings on the vehicle skin. The coating technique include applying a sheet material, spraying with powders at elevated temperatures, isostatic pressing, and coating with paints.
The disadvantage of the injection of the polymer into the boundary layer is that great quantities are used at high expense and recovery systems are not completely satisfactory. A disadvantage of the prior art coatings, no matter how applied, is their tendency to swell causing an increase in thickness, thus increasing the vehicle's size. Another disadvantage is that many coatings develop a flabby skin effect of a compliant coating. Both of these manifestations increasing drag.